Fibrin-Associated Large B-Cell Lymphoma (FA-LBCL) Involving Solid Organs as Necrotic Cystic Lesions—A Rare Entity with Potential Diagnostic Pitfalls: A Two-Case Series and Review of the Literature

Fibrin-associated large B-cell lymphoma (FA-LBCL) is a rare subtype of Epstein-Barr virus (EBV)-associated lymphoma, recognized as an independent entity per the 5th edition of the WHO classification of hematolymphoid neoplasms. It is usually associated with longstanding chronic inflammation and arises within fibrinous material in confined anatomic spaces. We report the clinicopathologic manifestations of two patients of FA-LBCL involving the adrenal gland and kidney. Both tumors were diagnosed after presenting as cystic masses on imaging studies. These lymphomas were non-invasive, with microscopic aggregates of large B-lymphoma cells along/within cystic wall and admixed with fibrinous material and without prominent inflammation. By immunohistochemistry and in-situ hybridization, lymphoma cells were positive for CD45, PAX5, CD79a, MUM1, BCL2, PD-L1, and EBV/EBER (Epstein-Barr virus encoded small RNA) with a high proliferation index. Both patients remain in remission after management with complete surgical resection and additional chemo-immunotherapy in one patient. Considering its rarity, scant tumor cells, and varied clinical presentations, FA-LBCL may pose diagnostic challenges, especially when presenting as extensively necrotic cystic lesions, needing multidisciplinary collaboration in formulating management.